1. Field of the Invention
This invention relates to a laser processing method and an apparatus of hole-boring, cutting or welding by irradiating objects (work pieces) with high power ultrashort pulse laser beams. Ultrashort pulse laser processing, which converges an ultrashort laser pulse by a lens and irradiates an object (a work piece) with the converged pulse, has an advantage of neither heating nor degenerating surrounding parts since heated regions are narrowly localized and ultrashort duration time gives no time of heat diffusion to the surrounding parts. The ultrashort pulse laser processing is a momentary, adiabatic processing. The ultrashort laser pulse processing is suitable for the processing of resins, glasses, quartz, sapphire, semiconductors, and so on.
This application claims the priority of Japanese Patent Application No. 2004-336944 filed on Nov. 22, 2004, which is incorporated herein by reference.
2. Description of Related Art
Japanese Patent Laying Open No. 2003-305585 “Laser processing and apparatus” pointed out a problem of the occurrence of color aberration caused by wavelength dispersion when an ultrashort pulse was converged by a refractive lens. Since the pulse width is very short, the wavelength does not take a constant value but fluctuates. Each lens material has an intrinsic refractive index dispersion (dn/dλ<0). The negative refractive index dispersion allocates shorter focal lengths to shorter wavelength rays and longer focal lengths to longer wavelength rays. Dispersion of the focal lengths degenerates convergence of the laser beam.
Japanese Patent Laying Open No. 2003-305585 proposed a hybrid lens having a refractive front surface and a diffractive rear surface as a lens of the ultrashort pulse laser processing. Diffractive lenses allocate longer focal lengths to shorter wavelength rays and shorter focal lengths to longer wavelength rays against refractive lenses. The function of a diffractive lens is contrary to the function of a refractive lens with regard to color aberration. Japanese Patent Laying Open No. 2003-305585 proposed a method of obtaining 0 color aberration by compensating the aberration induced by the refractive lens with the counter aberration caused by the diffraction lens. FIG. 1 shows a figure described in Japanese Patent Laying Open No. 2003-305585. The front surface (301A) is a refractive plane and the rear surface (301B) is a zigzag diffractive plane. The diffraction function cancels the color aberration caused by the refractive function in the optics in FIG. 1.